Spatio-temporal coupling of RMS errors in laser amplification.

We developed a theoretical model for the relationship between the input and amplified laser beams of energy stability and spatial uniformity in the amplification process. 10 Hz, 8 ns, 1064 nm Nd:YAG Q-switched resonator with Nd:YAG main amplifier was employed for the experiment. The theoretical model simulation and Frantz-Nodvik simulation were performed by utilizing the obtained beam image, acquired energy from the experiment, and stored fluence from the gain medium. The result indicated that the fluctuation of the spatial distribution in a single beam influences the stability of temporally distributed energy during the amplification process of the laser beam, thereby improving energy stability.

Simultaneous Measurement of Group Refractive Index Dispersion and Thickness of Fused Silica Using a Scanning White Light Interferometer.

In this study, we simultaneously measured the group refractive index dispersion and thickness of fused silica using a scanning white light interferometer on a spectral range from 800 to 1050 nm. A delay error correction was performed using a He-Ne laser. The accuracy of the measured group refractive index dispersion of fused silica, when compared to the temperature-dependent Sellmeier equation, is within 4 × 10-5.

Performance of zoom homogenizer to control the size of illumination field.

We performed an illumination simulation, wave-optical simulation, and experiment on a zoom homogenizer and conventional beam homogenizer. The wave-optical simulation was performed for each lenslet using the shifted angular spectrum method for effective memory use. In the experiment, the parameters of the zoom homogenizer and beam homogenizer at all zoom positions were set using a system matrix. The uniformity and peak effect of the beam homogenizer were 16.57% and 16.63%, respectively, but those of the zoom homogenizer were 7.15% and 1.46%, respectively. The illumination field generated by the zoom homogenizer had a higher uniformity and lower peak effect at all zoom positions than the conventional one.

Jones calculus modeling and analysis of the thermal distortion in a Ti:sapphire laser amplifier: erratum.

Equations 41-44 in our paper [ Opt. Express26, 14362 (2016)] contain errors. We correct the equations in this erratum.

Design of square-shaped beam homogenizer for petawatt-class Ti:sapphire amplifier.

We have designed a square-shaped beam homogenizer using a lens array for pumping a petawatt-class Ti:sapphire amplifier. The designed beam homogenizer generated a flat-top uniform square pump beam with 5.1% of edge steepness and 2.7% of RMS spatial uniformity, respectively. This pump beam generated an output beam with 74 J of energy and 56.8% of extraction efficiency, respectively. A scalar diffraction and two-dimensional amplification simulation were performed for optimization. The results indicate the possibility of improving an amplification efficiency and developing a compact square petawatt-class laser system.

Numerical extension of Frantz-Nodvik equation for double-pass amplifiers with pulse overlap.

We developed a new numerical simulation method to calculate the amplification of arbitrary laser pulses in double-pass amplifiers in which two polarization-orthogonal pulses inside the gain medium overlap. The proposed method agrees very well with the experimental results given in this work for the pulse energy and temporal shape.

Jones calculus modeling and analysis of the thermal distortion in a Ti:sapphire laser amplifier.

The mathematical modeling of an anisotropic Ti:sapphire crystal with a significant thermal load is performed. The model is expressed by the differential Jones matrix. A thermally induced distortion in the chirped-pulse amplification process is shown by the solution of the differential Jones matrix. Using this model, the thermally distorted spatio-temporal laser beam shape is calculated for a high-power and high-repetition-rate Ti:sapphire amplifier.

Enhancement of electron energy to the multi-GeV regime by a dual-stage laser-wakefield accelerator pumped by petawatt laser pulses.

Laser-wakefield acceleration offers the promise of a compact electron accelerator for generating a multi-GeV electron beam using the huge field gradient induced by an intense laser pulse, compared to conventional rf accelerators. However, the energy and quality of the electron beam from the laser-wakefield accelerator have been limited by the power of the driving laser pulses and interaction properties in the target medium. Recent progress in laser technology has resulted in the realization of a petawatt (PW) femtosecond laser, which offers new capabilities for research on laser-wakefield acceleration. Here, we present a significant increase in laser-driven electron energy to the multi-GeV level by utilizing a 30-fs, 1-PW laser system. In particular, a dual-stage laser-wakefield acceleration scheme (injector and accelerator scheme) was applied to boost electron energies to over 3 GeV with a single PW laser pulse. Three-dimensional particle-in-cell simulations corroborate the multi-GeV electron generation from the dual-stage laser-wakefield accelerator driven by PW laser pulses.

Transition of proton energy scaling using an ultrathin target irradiated by linearly polarized femtosecond laser pulses.

Particle acceleration using ultraintense, ultrashort laser pulses is one of the most attractive topics in relativistic laser-plasma research. We report proton and/or ion acceleration in the intensity range of 5×10(19) to 3.3×10(20) W/cm2 by irradiating linearly polarized, 30-fs laser pulses on 10-to 100-nm-thick polymer targets. The proton energy scaling with respect to the intensity and target thickness is examined, and a maximum proton energy of 45 MeV is obtained when a 10-nm-thick target is irradiated by a laser intensity of 3.3×10(20) W/cm2. The proton acceleration is explained by a hybrid acceleration mechanism including target normal sheath acceleration, radiation pressure acceleration, and Coulomb explosion assisted-free expansion. The transition of proton energy scaling from I(1/2) to I is observed as a consequence of the hybrid acceleration mechanism. The experimental results are supported by two- and three-dimensional particle-in-cell simulations.

Relativistic frequency upshift to the extreme ultraviolet regime using self-induced oscillatory flying mirrors.

Coherent short-wavelength radiation from laser-plasma interactions is of increasing interest in disciplines including ultrafast biomolecular imaging and attosecond physics. Using solid targets instead of atomic gases could enable the generation of coherent extreme ultraviolet radiation with higher energy and more energetic photons. Here we present the generation of extreme ultraviolet radiation through coherent high-harmonic generation from self-induced oscillatory flying mirrors--a new-generation mechanism established in a long underdense plasma on a solid target. Using a 30-fs, 100-TW Ti:sapphire laser, we obtain wavelengths as short as 4.9 nm for an optimized level of amplified spontaneous emission. Particle-in-cell simulations show that oscillatory flying electron nanosheets form in a long underdense plasma, and suggest that the high-harmonic generation is caused by reflection of the laser pulse from electron nanosheets. We expect this extreme ultraviolet radiation to be valuable in realizing a compact X-ray instrument for research in biomolecular imaging and attosecond physics.

Single-pulse coherent diffraction imaging using soft x-ray laser.

We report a coherent diffraction imaging (CDI) using a single 8 ps soft x-ray laser pulse at a wavelength of 13.9 nm. The soft x-ray pulse was generated by a laboratory-scale intense pumping laser providing coherent x-ray pulses up to the level of 10(11) photons/pulse. A spatial resolution below 194 nm was achieved with a single pulse, and it was shown that a resolution below 55 nm is feasible with improved detector capability. The single-pulse CDI might provide a way to investigate dynamics of nanoscale molecules or particles.

Generation of high-contrast, 30 fs, 1.5 PW laser pulses from chirped-pulse amplification Ti:sapphire laser.

High-contrast, 30 fs, 1.5 PW laser pulses are generated from a chirped-pulse amplification (CPA) Ti:sapphire laser system at 0.1 Hz repetition rate. The maximum output energy of 60.2 J is obtained, at a pump energy of 120 J, from a booster amplifier that is pumped by four frenquency-doubled Nd:glass laser systems. During amplification, parasitic lasing is suppressed by index matching fluid with absorption dye and the careful manipulation of the time delay between the seed and pump pulses. An amplified pulse passes through a pulse compressor consisting of four gold-coated gratings. After compression, the measured pulse duration is 30 fs, and the output energy is 44.5 J, yielding a peak power of about 1.5 PW. The output energy of 44.5 J and output power of 1.5-PW are the highest values ever achieved from the femtosecond CPA Ti:sapphire laser system. To maintain a sufficiently high temporal contrast, a saturable absorber is installed in the front-end system with two ultrafast Pockels cells in order to minimize the amplified spontaneous emission (ASE) and pre-pulse intensity. An adaptive optics system is implemented for PW laser pulses and a focused intensity of about 1 × 10(22) W/cm(2) can be obtained when an f/3 optic is used.

0.1 Hz 1.0 PW Ti:sapphire laser.

We report on the generation of 1.0 PW, 30 fs laser pulses at a 0.1 Hz repetition rate from a chirped-pulse amplification Ti:sapphire laser system. The energy of the laser pulses is amplified up to 47 J in a final three-pass booster amplifier having 96 J pump energy. To the best of our knowledge, this is the first petawatt Ti:sapphire laser system at a 0.1 Hz repetition rate. The shot-to-shot energy fluctuation of the laser pulses is as low as 0.53% in rms value, and the laser pulses have homogeneous flattop spatial beam profiles.

Long-term carrier-envelope-phase stabilization of a femtosecond laser by the direct locking method.

We have developed a practical solution to implement the direct locking method for the carrier-envelope phase (CEP) stabilization of femtosecond laser pulses and achieved 24-hour CEP stabilization without realignment of any optical components. The direct locking method realizes the CEP stabilization in the time domain by directly quenching the beat signal from an f-to-2f interferometer and, thereby, locking every pulse to a same CEP. We have accomplished the long-term CEP stabilization using commercially available standard feedback electronics, and maintained the CEP stabilization with low jitter without using any frequency-analyzing components, greatly facilitating the accessibility of the CEP stabilization.

Precise and long-term stabilization of the carrier-envelope phase of femtosecond laser pulses using an enhanced direct locking technique.

We demonstrate a long-term operation with reduced phase noise in the carrier-envelope-phase (CEP) stabilization process by employing a double feedback loop and an improved signal detection in the direct locking technique [Opt. Express 13, 2969 (2005)]. A homodyne balanced detection method is employed for efficiently suppressing the dc noise in the f-2f beat signal, which is converted into the CEP noise in the direct locking loop working at around zero carrier-envelope offset frequency (f(ceo)). In order to enhance the long-term stability, we have used the double feedback scheme that modulates both the oscillator pump power for a fast control and the intracavity-prism insertion depth for a slow and high-dynamic-range control. As a result, the in-loop phase jitter is reduced from 50 mrad of the previous result to 29 mrad, corresponding to 13 as in time scale, and the long-term stable operation is achieved for more than 12 hours.

100-kHz high-power femtosecond Ti:sapphire laser based on downchirped regenerative amplification.

We have generated femtosecond pulses with a peak power as high as 0.7 GW at the repetition rate of 100 kHz from a downchirped-pulse amplification (DPA) Ti:sapphire laser. For the high-energy amplification with high repetition rate, we employ a regenerative amplifier, acousto-optically switched and pumped by a Q-switched green laser. The DPA-based dispersion control is achieved up to the third-order term by use of a prism-grating stretcher and a glass compressor. We have obtained 28 muJ, 39 fs laser pulses with a compression efficiency of 95 %. Temporal and spatial characteristics of the laser pulses are investigated.

Tunable Q-switched erbium-doped fiber laser based on digital micro-mirror array.

We propose and demonstrate a tunable Q-switched erbium doped fiber laser with a digitally controlled micro-mirror array device. The tunable and pulsed output of the laser was achieved by the pixelated spatial modulation of the micro-mirror array. The wavelength tuning from 1530 nm to 1555 nm was shown with wavelength selectivity of ~0.1 nm and the pulsed operation was accomplished with 130 Hz repetition rate.